The invention relates to a method and a circuit assembly for controlling motor current in an electric motor with three or more phases, especially a stepper motor with three or more phases, by means of a chopper process.
With 2-phase motors, and especially 2-phase stepper motors, it is known by means of a chopper process from an added motor supply voltage for each of the two motor coils to generate current direction, current amplitude and current form to be injected (in a microstep operation normally a sine or cosine form) according to a current preset (coil target current) to thus drive the rotor of the motor by means of PWM current pulses.
With this three different coil current phases are distinguished, which are activated via the chopper process:
In the ON-phase, the coil current is driven into a coil in the direction of the instantaneous preset polarity or direction of the coil current actively via the coil, so that the amount of the coil current rises relatively quickly and continuously (switch-on period). The coil current direction injected through an ON phase is thus equal to the instantaneous polarity or direction of the coil current.
In the case of as sine-wave coil current, the coil current is for example positive in the first and second quadrants and negative in the third and fourth quadrants.
In the fast decay (FD) phase, the coil current is actively decomposed against the just-preset polarity of the coil current through reversal of the coil pole and feedback of the coil current, i.e., the coil current direction is set opposite to the polarity of the coil current. Especially in the phases of falling coil current amount (i.e., during the second and fourth quadrant of a sine wave shaped coil current), the FD phase serves for decomposing the coil current relatively quickly and to prevent the current preset from being adulterated especially via the counterelectromotive forces.
Designated as the third coil current phase of the chopper operation is the recirculation phase or slow decay (SD) phase, in which the coil is not actively controlled but rather short-circuited or bridged, so that the coil current, due to the inner resistance of the coil and the counter-EMF, drops off only gradually (i.e., slower than during the FD phases) in terms of amount.
These three coil current phases are temporally activated, determined and combined by the chopper process so that the de facto coil current over its entire (for example, sine-wave shaped) course, thus during, the rising and the falling current phases, follows a current preset (coil target current) as temporally close and precisely as possible, and especially is not (substantially) altered through the voltage (counter-EMF) counter-induced through the rotor in the motor coils. For this it is required to measure or to determine the de facto current through each of the two coils in a suitable manner.
However, in contrast to 2-phase stepper motors, with 3-phase stepper motors, control of the individual motor coils, each offset by 120° spatially in relation to the rotor in a manner electrically uncoupled from each other, is not possible (with only DE 10 2007 040 166 as an exception), since within the motor they are connected with each other in a triangular or star connection or otherwise, and thus not all (total of six) connections of all (three) coils can be guided outwards in a manner electrically insulated from each other. 3-phase stepped motors therefore are normally guided by a controlled chopper, which impinges on the three outer connections U, V, W of the motor that are available with PWM current pulses that are offset to each other as regards their phase by 120°, as is known in similar fashion for controlling 3-phase BLDC motors.
With such control, the individual coils of the motor then see an effective control voltage which corresponds to the difference between the temporal durations of the particular PWM current pattern at the two connections of the pertinent motor coil. Thus the motor is controlled with an effective control voltage and by this means can be rotated very precisely in the low speed range. For this purpose, the PWM current pulses are sine-wave modulated, so that a sine-wave current progression is generated in the motor coils, i.e., in the three connections U, V and W of the motor, currents are fed in that have a sine-wave shape and exhibit the phases sin(x), sin(x+120°) and sin(x−120°) relative to each other.
However, it has been shown that the motor sometimes exhibits very rough running as soon as the r.p.m. of the motor during such operation comes close to the mechanical (natural) resonant frequency of the motor. Evidently one substantial cause is that the counter-EMF of the motor that arises through the resonances and is no longer sine-wave-shaped, can come into the order of magnitude of the control voltages on the motor coils and partially cancel them. This counter-EMF arising through the resonance-caused oscillations is subtracted from the control voltages, so that the resulting coil currents can vary substantially from the sine-wave progression necessary for an optimal motor operation. Since the rotor of the motor is controlled by the magnetic fields and thus by the coil currents, it can in turn no longer follow the preset sine-wave-shaped control voltages with sufficient precision.
It is in fact conceivable by means of a regulating device such as a PID regulator to readjust the control voltages on the motor coils in dependence on the counter-EMF, so that the coil currents remain as close to sine-wave-shaped as possible. However, in addition to higher circuit costs and its greater complexity, use of such a higher-order controller has a disadvantage in that it is critical to set parameters for the controller, and if necessary, motor-specific parameters are required or are dependent on it.
It is desirable to provide a method and a circuit assembly for controlling the motor current in an electrical 3-phase motor, especially a 3-phase stepper motor, by which the above-mentioned disadvantages are avoided, and especially not requiring a higher-order controller for the above-mentioned purpose.
It is also desirable to provide a method and a circuit assembly for controlling the motor current in an electrical 3-phase motor, especially a 3-phase stepper motor, by which the motor runs smoothly, even at higher and high motor r.p.m. with relatively low additional circuit complexity.
According to an aspect of the invention, a method is provided for controlling the motor current in an electrical 3-phase motor, especially a 3-phase stepper motor, with a first, a second and a third motor connection, wherein a first, a second and a third chopper phase are activated in cyclically alternating fashion, during each of which the motor is connected between a supply voltage and ground, namely, on the one hand by means of one of the first, the second and the third motor connection and on the other hand by means of the two other of the first, the second, and the third motor connections, wherein the latter are connected with each other, and wherein during at least two of these three chopper phases, a target current supplied for the affected one of the first, the second, and the third motor connection is injected into this one motor connection by means of a chopper process.
According to another aspect of the invention, a circuit assembly is provided for controlling the motor current in an electrical 3-phase motor, especially a 3-phase stepper motor, with a first, a second and a third motor connection, which circuit assembly exhibits a driver circuit (DR) controlled by a chopper (CH), and by which circuit assembly a cyclic alternating switching takes place between at least two chopper phases, wherein during each chopper phase into each one of the motor connections, by means of a chopper process, a motor current is injected which corresponds to a fed motor target current for this motor connection, wherein the two other motor connections are connected with each other via the driver circuit (DR).
With the invention-specific method and the invention-specific circuit assembly, robust and simple control of motor currents is possible, and due to constant current operation, motor natural resonances can be efficiently dampened.
Due to the simplicity of the method, expense for digital switching elements and components for signal detection is relatively low.
Therefore, the invention-specific circuit assembly is suitable for integration especially in such integrated circuits as must employ high-voltage technologies to make possible direct control of motors or of power transistors which in turn control a motor. Due to their great structural width, such integrated circuits have only limited possibilities for economical integration of large digital components. Thus, a suitable complete module can be implemented for mass production as well.